4 PULSE-ECHO pulse-echo: refers to an ultrasound system that is used for ultrasound imaging by periodically transmitting and receiving short duration sound pulses A pulse-echo system is used for ultrasound imaging. A transducer that is in contact with the patient is periodically supplied with electrical pulses. The transducer converts the electrical pulses into short bursts of mechanical sound energy that are transmitted into the patient.

5 PULSE-ECHO acoustic interface: the reflecting boundary that is formed between two materials with different acoustic impedance characteristics The transmitted sound encounters numerous reflectors, which are also called acoustic interfaces. The interfaces are reflecting boundaries formed by changes in tissue characteristics, such as from soft tissue to blood or fat to muscle. These changes produce echoes that return to the transducer. By keeping track of the transit time, which is the time between transmitted pulses and returning echoes, the ultrasound system learns the locations of the various interfaces. Once the locations are known, the echoes are positioned on a display to form image patterns that reproduce the patient s internal structures.

6 RANGE EQUATION: (in tissue) Pulse-echo ultrasound systems use the range equation to determine the locations of the various interfaces. In soft tissue, sound travels 6.5 microseconds per centimeter. The distance to an interface (in millimeters) equals 0.77 multiplied by the round trip time of the sound pulse (in microseconds). TIME TO THE REFLECTOR (ONE WAY) DISTANCE TO THE REFLECTOR (ONE WAY) ROUND TRIP TIME ROUND TRIP DISTANCE 6.5 µs 10 mm (1 cm) 13 µs 20 mm (2 cm) 5 µs 7.7 mm (0.77 cm) 10 µs cm (1.54 mm) The range equation assumes a velocity (speed of sound) of 1540 meters per second. Variations in the speed of sound have an effect on the distance accuracy of a pulse-echo system, resulting in axial errors.

7 PULSE-ECHO EQUATIONS Pulse Repetition Period = 1 Pulse Repetition Frequency For pulse-echo imaging, the rate at which pulses are transmitted is the pulse repetition rate or pulse repetition frequency (PRF). The PRF is typically greater than 1000 Hz (1kHz) and is often a function of the ultrasound system s DEPTH control. The result of a high PRF is a short pulse repetition period (PRP), which is the interval between the start of successive pulses. When the PRP is too short, a new pulse may be transmitted before the return of deep echoes from the previously transmitted pulse. Deep echoes will be incorrectly displayed. The result is range ambiguity, or depth ambiguity. This decreases the maximum depth that can be accurately imaged. PRF PRP 1000 Hz 1/1000 sec (0.001 sec) 2000 Hz 1/2000 sec ( sec) 4000 Hz 1/4000 sec ( sec)

9 ACOUSTIC IMPEDANCE: Acoustic Impedance = Density x Velocity The change in a tissue characteristic that creates an acoustic interface is acoustic impedance, which is specific for each type of tissue. The acoustic impedance of a material may be determined mathematically by multiplying the material s density by the velocity of sound in the material. Example: water has a density of 1000 and a sound velocity of 1430, which produces an acoustic impedance of 1,430,000. Acoustic impedance is expressed in rayls. DENSITY VELOCITY ACOUSTIC IMPEDANCE Increase Increase Decrease Decrease Increase Increase Decrease Decrease

11 INTERFACE MATERIALS & ECHO STRENGTH Soft Tissue to Muscle - Weak (1%) Fat to Soft Tissue Weak (1%) Soft Tissue to Bone - Medium (50%) Blood to Plaque Medium (50%) Soft Tissue to Air - Very Strong (100%) The greater the disproportion between the acoustic impedance values of the two materials forming an interface, the greater the strength of the returning echo. Weak interfaces have echo reflection coefficients in the 0.01 (1%) range. Medium strength interfaces have echo reflection coefficients in the 0.5 (50%) range. Very strong interfaces have echo reflection coefficients in the 1.0 (100%) range. The transmission coefficient is equal to 1 minus the reflection coefficient.

12 SAGITTAL - LIVER, RIGHT KIDNEY ACOUSTIC SHADOW: THE RESULT OF A STRONG REFLECTION FROM A GALLSTONE contrast agents: tiny, highly reflective, encapsulated gas filled microbubbles or solid particles, which are injected into the blood stream and return echoes that are thousands of times more reflective than blood A material that produces a large acoustic impedance change, which results in a strong reflection, causes a significant reduction in the amount of sound that is available for further transmission through the patient. This may produce an acoustic shadow, which appears as an absence of displayed echoes for structures that are located beneath the interface. Bone, gallstones, calcified plaque, and bowel gas and some image enhancing ultrasound contrast agents can produce acoustic shadows, which fall into one of the categories of ultrasound artifacts.

13 ACOUSTIC COUPLANT Due to the high reflectivity of air, the use of an acoustic couplant is essential. An acoustic couplant is a liquid (oil or gel) that is placed on a patient to insure good contact between the transducer and the skin and to diminish the amount of air that is normally present.

15 SPECULAR REFLECTION Small change or no change in velocity With i normal or oblique, t indicates no refraction Reflections (echoes) most frequently received are those that occur at normal incidence, which is when the interface is perpendicular to the ultrasound beam. Anything other than normal incidence is oblique incidence. According to the Law of Reflection, the angle of incidence, or insonating angle, ( i ) equals the angle of reflection ( r ). Since these two angles are equal, the transducer does not detect reflections that are produced as a result of oblique incidence.

16 SPECULAR REFLECTION Small change or no change in velocity With i normal or oblique, t indicates no refraction Because of oblique incidence, every structure under the transducer is not always visible in an ultrasound display. This is particularly true if the interface is specular (smooth and large compared to the size of an ultrasound wave). Examples of specular interfaces are the diaphragm, the walls of vessels and cystic structures, and the boundaries of many organs.

17 SPECULAR REFLECTION Large increase in velocity With i oblique, refraction is present As sound is transmitted through a specular interface, refraction (as expressed by Snell s Law) may be present. Refraction, which is a change in the direction of the waves transmitted through an interface, is the result of oblique incidence and a significant difference in the velocities of sound in the two materials forming the interface. The transmitted angle ( t ) will be larger than the incident angle in proportion to an increase in the respective velocity.

18 SPECULAR REFLECTION Large decrease in velocity With i oblique, refraction is present The transmitted angle ( t ) will be smaller than the incident angle in proportion to a decrease in the respective velocity. Since ultrasound scanners assume all echoes originate along the original axis of the transmitted beam, refraction can occur without the knowledge of the sonographer. Refraction may cause improper lateral positioning of displayed echoes.

19 SAGITTAL - LIVER, RIGHT KIDNEY N = non-specular reflector S = specular reflector 1 = diaphragm 2 = kidney Interfaces that are either smaller than a wavelength, or not smooth are nonspecular. While specular interfaces reflect sound in only one direction, nonspecular interfaces produce scattering of the sound. Examples of non-specular interfaces include red blood cells, some microbubble contrast agents, liver parenchyma, and many other tissue-like structures. Interfaces that are smaller than a wavelength produce Rayleigh scattering. This type of scattering of sound accounts for the texture that is displayed in an ultrasound image.

20 Answers to the following FOURTEEN practice questions were derived from material in the textbook:

23 Question 2 If the number of cycles in a pulse is reduced, the pulse repetition frequency is automatically reduced the spatial pulse length increases the period increases the duty factor is smaller the bandwidth is decreased Pages 6, 7, and 9

24 Question 2 If the number of cycles in a pulse is reduced, the pulse repetition frequency is automatically reduced the spatial pulse length increases the period increases the duty factor is smaller the bandwidth is decreased Pages 6, 7, and 9

25 Question 3 Damping in a transducer reduces the transducer's resonant frequency increases the number of cycles in a pulse causes poor axial and lateral resolution reduces the number of cycles in a pulse Pages 5, 6, 7, and 9

26 Question 3 Damping in a transducer reduces the transducer's resonant frequency increases the number of cycles in a pulse causes poor axial and lateral resolution reduces the number of cycles in a pulse Pages 5, 6, 7, and 9

27 Question 4 The distance to a target is doubled. The total time for a pulse to travel to the target and back is 4 times 2 times 8 times the same halved Page 8

28 Question 4 The distance to a target is doubled. The total time for a pulse to travel to the target and back is 4 times 2 times 8 times the same halved Page 8

29 Question 5 If ultrasound energy leaves a transducer and travels through a large amount of fat and then encounters a reflector, the echo will appear on the display to the right of where it should more superficial than it should deeper than it should to the left of where it should in its correct position Pages 2 and 8

30 Question 5 If ultrasound energy leaves a transducer and travels through a large amount of fat and then encounters a reflector, the echo will appear on the display to the right of where it should more superficial than it should deeper than it should to the left of where it should in its correct position Pages 2 and 8

31 Question 6 The reason most ultrasound systems are calibrated at 1540 meters per second is because 1540 meters per second is the average speed of sound in the patient 1540 meters per second is the speed of sound most often encountered in a patient 1540 meters per second is the maximum speed of sound in a patient 770 meters per second is the average speed of sound in a patient 1540 meters per second is the minimum speed of sound in a patient Pages 2 and 8

32 Question 6 The reason most ultrasound systems are calibrated at 1540 meters per second is because 1540 meters per second is the average speed of sound in the patient 1540 meters per second is the speed of sound most often encountered in a patient 1540 meters per second is the maximum speed of sound in a patient 770 meters per second is the average speed of sound in a patient 1540 meters per second is the minimum speed of sound in a patient Pages 2 and 8

35 Question 8 Without a perpendicular angle of incidence at a non-specular reflector the bandwidth will increase it is not likely that an echo will return to the transducer Rayleigh scattering is not possible it is possible that an echo will return to the transducer the transmission coefficient will exceed 100% Pages 12, 13, and 14

36 Question 8 Without a perpendicular angle of incidence at a non-specular reflector the bandwidth will increase it is not likely that an echo will return to the transducer Rayleigh scattering is not possible it is possible that an echo will return to the transducer the transmission coefficient will exceed 100% Pages 12, 13, and 14

37 Question 9 If the velocities of sound in the two materials that form an interface are NOT equal, the reflection coefficient will exceed 1 the reflected angle will be greater than the incident angle the transmission coefficient will exceed 1 refraction may occur if the incident angle is not perpendicular the incident angle will be greater than the reflected angle Pages 12 and 13

38 Question 9 If the velocities of sound in the two materials that form an interface are NOT equal, the reflection coefficient will exceed 1 the reflected angle will be greater than the incident angle the transmission coefficient will exceed 1 refraction may occur if the incident angle is not perpendicular the incident angle will be greater than the reflected angle Pages 12 and 13

39 Question 10 The redirection of sound energy in many directions as a result of a rough boundary between two media is shadowing specular reflection through-transmission scattering refraction Page 14

40 Question 10 The redirection of sound energy in many directions as a result of a rough boundary between two media is shadowing specular reflection through-transmission scattering refraction Page 14

41 Question 11 The density is the same in materials A and B, but the speed of sound in material B is 10% greater than the speed of sound in material A. The acoustic impedance in B is 10% greater than the acoustic impedance in A The sound velocity in A is 10% higher than the propagation speed in B The acoustic impedance in A is equal to the acoustic impedance in B The acoustic impedance in A is 10% higher than the acoustic impedance in B The acoustic impedance in B is 2 times the acoustic impedance in A Page 10

42 Question 11 The density is the same in materials A and B, but the speed of sound in material B is 10% greater than the speed of sound in material A. The acoustic impedance in B is 10% greater than the acoustic impedance in A The sound velocity in A is 10% higher than the propagation speed in B The acoustic impedance in A is equal to the acoustic impedance in B The acoustic impedance in A is 10% higher than the acoustic impedance in B The acoustic impedance in B is 2 times the acoustic impedance in A Page 10

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